Copolymers of styrene and maleic anhydride (SMA) are known to have good heat resistance and are widely used in commercial applications requiring good performance at elevated temperatures. The thermal stability and weatherability of the SMA copolymers are further improved by converting the anhydride groups to the corresponding imide groups by reacting with aliphatic and aromatic primary amines, either in solution or in the melt (reactive extrusion), and by direct polymerization of N-substituted or unsubstituted maleimides.
A similar effort has been made to convert poly(methyl methacrylate)- or polymethacrylic anhydride-containing polymers to their corresponding N-substituted methacrylimides to improve thermal stability, processability, strength and modulus, as well as weatherability.
In general, polymers containing maleimide and methacrylimide groups are found to be miscible with a number of engineering thermoplastics. In particular, styrene/N-phenylmaleimide copolymers are finding commercial applications in blends with ABS to improve the thermal stability, melt flow rate and heat distortion temperatures and the ability to mold a thinner gauge material with reduced cycle times. These thermally resistant ABS grades are expected to expand markets for ABS by effectively competing with other engineering thermoplastics such as PPE. Due to the improved compatibility of the ABS blends with a wide range of thermoplastics, it is expected to be of interest to enhance the thermal resistance of PVC, polycarbonate, and polyamides.
Aniline, a primary aromatic amine, is a hazardous carcinogenic liquid. It is difficult to handle aniline safely during reactive extrusion, which is the most economical way to produce imides on a large scale. Often maleic anhydride is reacted with aniline to obtain the N-phenylmaleimide monomer, which is then copolymerized with other monomers in solution or emulsion polymerization processes.
For example, U.S. Pat. No. 3,998,907 discloses a method for producing rubber-modified styrene/N-phenylmaleimide in aqueous suspension by heating a mixture of styrene/maleic anhydride, an elastomer and aqueous ammonia or an aqueous amine such as aniline, ethanolamine, benzylamine, and alkyl amines. U.S. Pat. No. 4,742,123 discloses non-crosslinked polymer compositions having glutaric anhydride units made by devolatilizing a (meth)acrylic acid-containing polymer or copolymer in an extruder to form the anhydride and then adding ammonia, an ammonia generator or a primary amine. U.S. Pat. No. 5,478,903 discloses the production of N-phenylmaleimide-containing copolymers in solution by direct copolymerization of N-phenylmaleimide with other monomers and organic peroxide, followed by solvent removal. U.S. Pat. No. 4,246,374 describes a method for making imidized acrylic polymers by reacting a polymer containing units derived from esters of acrylic or methacrylic acid with ammonia or a primary amine such as aniline, p-aminobenzoic acid or an alkylamine under anhydrous conditions in an extruder while applying subatmospheric pressure.
U.S. 4,891,405 discloses moldable blends of styrene-maleic anhydride copolymer, polybutylene terephthalate ester, and styrene-maleimide copolymer having pendant carboxyl or hydroxyl groups. The styrene-maleimide copolymer with pendant groups is made by modification of a styrene-maleic anhydride copolymer with an amino acid or ester by refluxing with azeotropic removal of water in a solvent.
There is still a need for a simpler and safer method to produce copolymers containing N-phenylsuccinimide or N-phenylmethacrylimide units by reactive extrusion without employing hazardous materials such as aniline.